1. Field of the Invention
The present invention relates to oriented polyamide fiber which has a high strength, modulus of elasticity and roundness and which is useful for use in sporting goods such as strings for a racket, in industrial materials such as a rubber reinforcing material and a filter cloth material for paper making and so forth; and a process for producing said polyamide fiber.
2. Description of the Related Arts
Polyamide fiber is generally employed in sporting goods such as strings for a racket, in industrial materials such as a rubber reinforcing material and a filter cloth material for paper making and so forth. In the above-mentioned purposes of use, the polyamide fiber as the construction material is required to be imparted with sufficient modulus of elasticity, that is, Young's modulus and also sufficient mechanical strength including loop strength and knot tensile strength. Moreover in said purposes of use, the polyamide fiber is required to be high in its roundness in the case where a final product made from the fiber is put into practical use, or in the case of secondary processing
Specifically, when the polyamide fiber is low in its roundness, it will bring about such problems that it is hard to pass the fiber through a clearance or an opening of an accurately processed article, thus lowering the adaptability thereof to machinery and equipment for secondary processing and that the shape of its product after secondary, processing is not uniformized.
A polyamide having repeated units of amide bond which is obtained from m-xylylenediamine and an aliphatic dicarboxylic acid (for example, a polyamide obtained from m-xylylenediamine and adipic acid, hereinafter sometimes referred to as "polyamide MXD6") is expected to find applications in the above-mentioned purpose of use, since it is characterized by its high strength, high Young's modulus and the like as compared with the conventional polyamide 6 and polyamide 66. However, the polyamide MXD6 fiber having high roundness can not be produced with conventional spinning methods, which becomes an obstacle to its practical application.
In more detail, a melt spinning method is usually applied to the production of the polyamide fiber which is employed in sporting goods such as strings for a racket, in industrial materials such as a rubber reinforcing material and a filter cloth material for paper making and so forth. Specifically there is adopted to the production, a spinning method in which a polyamide resin is molten with a single-screw or twin-screw extruder, the molten resin is spun through a spinneret, the spun product is pulled in a coolant bath placed beneath the face of the spinneret to produce non-oriented fiber, and thereafter the non-oriented fiber is oriented. For example, in the case of polyamide 6 or polyamide 66, by the use of non-oriented fiber in which crystallization is suppressed by setting the temperature of a coolant bath lower than the glass transition temperature (Tg) of the polyamide by at least 30.degree. C., it is facilitated to carry out orientation procedures while preserving the roundness of the non-oriented fiber, whereby the roundness of the fiber is enhanced. The above-mentioned method is exclusively adopted for the purpose of enhancing the roundness of fiber.
On the other hand, in the case of a polyamide containing polyamide MXD6, the Tg of the polyamide MXD6 is remarkably high as compared with that of polyamide 6 or polyamide 66. Therefore, solidification of the non-oriented fiber due to quenching rapidly takes place at a cooling temperature in the conventional melt spinning method for polyamide and at the same time, the resistance in a cooling vessel between the non-oriented fiber and the coolant, and the vibration of the fiber at the time of pulling the fiber in the bath bring about fiber swinging because of the high modulus of elasticity inherent to the polyamide MXD6, whereby the fiber swinging is likely to be transferred to the molten portion of the fiber with a lower strength thus causing a decrease in fiber roundness as well as diametral unevenness of fiber. For this reason, it has heretofore been extremely difficult to steadily and continuously produce a polyamide-MXD6-containing polyamide fiber with high roundness.